In the field of optical systems wherein a lens is utilized for magnifying certain information, it is important that the lens be positioned and maintained in such position to provide proper focusing of the lens for reading the information. In the case of a projection-type lens, it is advisable to use a high-efficiency lamp along with a high-resolution lens to obtain optimum operating conditions.
Since proper focusing of the optical system is extremely important, various ways and means have been utilized for moving the lens along the axial direction in relation to an optical plane. One well-known method is to adjust the position of the lens along its optical axis by camming means which includes structure whereby the lens and associated barrel of the lens system are axially moved in relation to the film or focal plane. Another method may use bevel surfaces in the nature of a helical curve whereby a pin or like member is caused to be moved along the curve for adjusting the lens in a manner to provide proper focusing thereof. In a number of these systems, it is usually required to re-focus the lens system upon reading of different information on microfilm or the like or when such film is caused to be replaced in film viewing apparatus.
Additionally, a lens may be positioned and retained in a system which includes structure for securing the lens in a manner to avoid repeated refocusing of the lens and such structure may be designed to make fast and simple changing of the lens which is then in the apparatus for a lens of a different magnification. Lens mounting systems have included bayonet-type structures but also use intricate machining to obtain a spring-effect to hold the lens against a defined surface as is the case in many of the lens mountings for cameras.
In the case of a viewer or reader for accommodating microfilm or microfiche, it is well-known that the reduction ratio of such readers may vary, for example, from 24 to 150 times and also the image size on the microform itself may vary. Because of these variations, it becomes advisable to have available a number of lenses of different magnification values rather than a plurality of readers with different reduction ratios.
The use of different magnification lenses in a specific reader or like apparatus presents a problem of maintaining focus when the lenses are interchanged to accommodate a microfiche or microfilm of like ratio. Additionally, it becomes necessary to provide structure which enables changing or replacing lenses in the shortest possible time. A plurality of lenses may be stored near the apparatus of the apparatus itself may carry the lenses in a manner whereby a specific lens may be selected from a group of lenses.
Representative prior art of selecting a lens from a lens-carrying member includes U.S. Pat. No. 3,640,621, issued to R. Frerson et al. on Feb. 8, 1972, which discloses apparatus wherein a plurality of lens systems are mounted on a turret for positioning any one of them to focus the image of a microform onto a screen or the plane of a copier. A servo mechanism is provided for positioning any one of the lens systems.